Field of the Invention
The present invention relates to flat display screens. It more particularly applies to electric connections to a vacuum airtight chamber of a flat display screen on device limited by two plates constituting the bottom and the viewing surface of the screen, respectively.
Conventionally, a flat display device is constituted by two external rectangular plates, for example made of glass. These two plates are assembled with a sealing joint and are spaced one from the other. For a Field Effect Display (FED), a microtip display, or a Vacuum Fluorescent Display (VFD), the space between the two glass plates is evacuated, whereas for a plasma display, this space is filled with a low pressure gas.
FIGS. 1A and 1B schematically represent a microtip flat display device which constitutes an exemplary device to which the present invention can apply. FIG. 1B is a cross-sectional view along line B--B' of the rod view represented in FIG. 1A.
Such a microtip display basically comprises a microtip cathode and a grid provided with holes facing the microtips. The cathode faces a cathodoluminescent anode having a glass substrate 1 that constitutes the display surface. For the sake of simplification, details of the cathode, grid and anode in the useful surface of the display, are not represented in FIGS. 1A and 1B.
An example of the operation mode and the detailed structure of such a microtip display is disclosed in U.S. No. Pat. 4,940,916 assigned to Commissariat a l'Energie Atomique.
The cathode is divided into columns and is constituted, on a glass substrate 2, by cathode conductors made of a conductive layer arranged according to lattices. The microtips are formed on a resistive layer deposited over the cathode conductors and are disposed inside the lattices defined by the cathode conductors. The cathode is associated with the grid that is divided into rows. The intersection of a grid row and a cathode column defines a pixel.
This device uses the electric field generated between the cathode and the grid so that electrons are extracted from the microtips toward the phosphor elements of the anode. In a color display, the anode is provided with alternate strips of phosphor elements, each corresponding to a color (blue, red, green). The strips are mutually separated by an insulator. The phosphor elements are deposited over electrodes, constituted by corresponding strips of a transparent conductive layer such as indiumtin oxide (ITO). The groups of blue, red, and green strips are alternatively polarized with respect to the cathode, so that the electrons extracted from the microtips of a pixel of the grid/cathode are selectively directed toward the opposed phosphor elements of each color.
The cathode/grid and the anode are fabricated separately on the two substrates, or plates, 2 and 1. Then, they are assembled with a peripheral sealing joint 3. A vacuum chamber 4 is provided between the two plates to allow the electrons issued from the cathode to flow toward the anode.
The operation of such a display requires the provision, outside chamber 4, of electric connections from the display to an electronic control system to allow the adequate polarization of the cathode columns, grid rows and anode strips. In the case of a display with a switched anode and with a matrix addressing of the cathode/grid, the number of connections corresponds to the sum of the number of columns of the cathode conductors, of the number of grid rows and of the number of groups of phosphor element strips. For a monochrome display a single anode connection is required
To achieve these connections outside chamber 4, the columns of cathode conductors 5 and the grid rows 6 are conventionally extended outside the useful surface of the display on the inner surface of plate 2. Similarly, electric interconnection paths (not shown) of the groups of phosphor element strips are extended on the inner surface of plate 1 outside its useful surface. The useful surface of the display is represented in FIG. 1A in dotted lines 7.
The two plates 1 and 2 are assembled together and are shifted so that the ends of conductors 5 and 6 and the interconnection paths of the phosphor elements are accessible from outside. In other words and as illustrated in FIG. 1A, the plate 2 is larger than plate 1 and the plates are assembled, one of the edges only being aligned.
A drawback of conventional displays is that the protrusions 8 and 9 on plate 2 on two sides of the display for the cathode and grid connections, and on plate 1 on one side of the display for the anode connections constitute weak areas once the display is fabricated.
It has been devised to achieve, in the useful surface of the cathode/grid plate, crossing conductive areas to transfer on the outer surface of this plate row and column contacting points, thereby suppressing the plate protrusions. The implementation of such a method requires the use of complex tools to pierce numerous holes in the cathode/grid plate. In addition, the defective filling of these holes can cause failure of the tightness of the inner chamber 4, thus polluting the inner vacuum, which is detrimental for the lifetime of the display.
Another drawback of conventional displays lies in that the electric connections cannot be grouped on the same surface of the display, which complicates the subsequent coupling of these connections with an electronic control system.